Layered oxide materials for batteries

ABSTRACT

Materials are presented of the formula: 
     A x M y M i   zi O 2−d , where A is sodium or a mixed alkali metal including sodium as a major constituent;
         x&gt;0;   M is a metal or germanium;   y&gt;0;   M i , for i=1, 2, 3 . . . n, is a transition metal or an alkali metal;   z i ≧0 for each i=1, 2, 3 . . . n;   0&lt;d≦0.5;   the values of x, y, z i  and d are such as to maintain charge neutrality; and   the values of x, y, z i  and d are such that x+y+Σz i &gt;2−d.       

     The formula includes compounds that are oxygen deficient. Further the oxidation states may or may not be integers i.e. they may be whole numbers or fractions or a combination of whole numbers and fractions and may be averaged over different crystallographic sites in the material. Such materials are useful, for example, as electrode materials in rechargeable battery applications. Also presented is a method of preparing a compound having the formula A x M y M i   zi O 2−d .

TECHNICAL FIELD

The present invention related to electrodes that contain an active material comprising Layered oxide materials which show novel anion stoichiometry, and to the use of such electrodes, for example in a sodium ion battery application. The invention is also related to certain novel materials and to the use of these materials for example as an electrode material.

BACKGROUND ART

The lithium-ion rechargeable battery technology has received a lot of attention in recent years and is used in most electronic devices today. Lithium is not a cheap or abundant metal and it is considered to be too expensive to some markets requiring rechargeable battery technologies; such as large scale stationary energy storage applications. Sodium-ion batteries are similar to lithium ion batteries, both are rechargeable and comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na⁺ (or Li⁺) ions de-intercalate from the cathode and insert into the anode. Concurrently charge balancing electrons pass from the cathode through an external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.

Sodium ion battery technology is considered to offer certain advantages over lithium. Sodium is more abundant than lithium and some researchers think that this will offer a solution for a low cost and durable energy storage requirement, particularly for large scale applications such as grid level energy storage. Nevertheless a significant amount of development is required before sodium-ion batteries are a commercial reality.

There are a number of material types which have been shown to be useful in rechargeable sodium ion batteries which include; Metallate materials, Layered oxide materials, polyanionic compounds, phosphates and silicates. However, one of the most attractive classes of material is that of the layered oxides.

A well-known layered oxide material has the formula NaNi_(0.5)Mn_(0.5)O₂. In this material the transition metal nickel is present as Ni²⁺ while the manganese is present as Mn⁴⁺. This is an ordered material with the Na and Ni atoms residing in discrete sites within the structure. In this case the nickel ions (Ni²⁺) are a redox active element which contributes to the reversible specific capacity and the manganese ions (Mn⁴⁺) play the role of a structure stabilizer. Similarly, the compound NaNi_(0.5)Ti_(0.5)O₂ is analogous to NaNi_(0.5)Mn_(0.5)O₂ in that the Ni²⁺ ions provide the active redox centre and the Ti⁴⁺ ions are present for structure stabilization. There is a considerable quantity of literature describing NaNi_(0.5)Mn_(0.5)O₂ and the titanium analogue, as a precursor for the lithium layered oxide material LiNi_(0.5)Mn_(0.5)O₂ and NaNi_(0.5)Ti_(0.5)O₂ and the subsequent ion exchanging the sodium for lithium. However, recent electrochemical studies reported by Komaba et al Adv. Funct. Mater. 2011, 21, 3859 describe the sodium insertion performance of hard-carbon and layered NaNi_(0.5)Mn_(0.5)O₂ electrodes in propylene carbonate electrolyte solutions. The results obtained show that NaNi_(0.5)Mn_(0.5)O₂ exhibits some reversible charging and discharging ability, unfortunately however the capacity of the material fades by 25% or more, after only 40 cycles which makes the use of this material extremely disadvantageous for rechargeable energy storage applications. As such there is significant interest in improving the electrochemical performance of such materials.

This invention discloses an oxygen non-stoichiometric material compositions based on a layered oxide framework. Herein, we define oxygen non-stoichiometry as deviation from the ABO₂ formula of the layered oxide framework, wherein in a pristine material the ratio of elements is 1:1:2. Within this invention we claim that the ratio deviates from the ideal ABO₂ stoichiometry in the following manner ABO_(2-δ) wherein, the average oxidation state of one or more elements contained within the B site reduces to rebalance the structures charge and retain charge neutrality whilst the proportion of elements on the B site remains unchanged.

This yields the same atomic ratios within the A and B site only with variation on the O site. So the relative proportions of elements within the material can be expressed as 1:1:2_(−δ) and the average oxidation state of each site can be expressed as +1:+3_(−2/δ):−4⁻⁶⁷

Most of the background literature for these types of materials is based upon stoichiometric sodium transition metal oxides which adopt either a P2 or O3 layered structure. This invention describes novel compositions based on the sodium layered oxides for application in a sodium ion battery. Within this material class there is substantial prior art based on material composition, focused on the Na content and ratios of transition metal elements.

For example, US20070218361 A1 describes a material suitable for a sodium ion battery and claim a positive electrode based on a sodium-containing transition metal oxide. The compositions of the oxide is described as NaaLibMn_(x)My0_(2±c), where, M may include at least one selected from the group consisting of iron, cobalt, and nickel, a is in the range 0.6 to 1.1 and b may range from 0 to 0.5, the sum of x and y may range from 0.9 to 1.1, and c may be from 0 to 0.1. In this application it is claimed that the number of oxygen atoms in the materials formula unit is based on charge compensation of the structure. In this compositional claim the O content is linked to the Na/Li content of the material and cannot be deficient i.e. there cannot be fewer oxygen atoms in the material than the sum of alkali and transition metals.

Similarly, in U.S. Pat. No. 8,835,041B2 a layered oxide material suitable for application in an energy storage device is defined. The compositional claims of this patent are described by the formula Na_(c)Li_(d)Ni_(e)Mn_(f)M_(z)O_(b), wherein M comprises one or more metal cations, and the ratios of constituents are limited to 0.24<c/b=<0.5, 0<d/b=<0.23, 0=<e/b=<0.45, 0=<f/b=<0.45, 0=<z/b=<0.45, the combined average oxidation state of the metal components is in the range of about 3.9 to 5.2, and b is equal to (c+d+Ve+Xf+Yz)/2, wherein V is the average oxidation state of the Ni, X is the average oxidation state of the Mn, and Y is the average oxidation state of the M in the material. M may be selected from Mg2+, Co2+, Co3+, B3+, Fe2+, Fe3+, Ga3+, A13+, and Ti4+.

Further to these disclosures, JP2012252962A claims a positive electrode for a secondary battery, expressed by the formula: Li_(a)A_(b)M_(c)O_(d), where A represents one or more elements selected from the group consisting of Na and K, M represents one or more transition metal elements, 0<a≦1.5, 0<b≦1. 5, 0<c≦3, 0<d≦6, and 0<a+b≦1. 5. The patent refers to these materials adopting a spinel type structure rather than a layered oxide structure.

Further to these documents,

U.S. Pat. No. 8,709,655B2, JP5085032B2, US20090159838A1, WO2006/057307, JP2006179473A, US20110003192A1, and WO2009/099061 detail the application of sodium layered oxides for application in energy storage devices The compositional claims of these documents limit the content of oxygen in the material to O2 and they do not contain any oxygen non stoichiometry.

SUMMARY OF INVENTION

To the accomplishment of the foregoing and related ends, the invention, comprises the features hereinafter fully described and identified in the claims. The following description and the annexed drawings set forth detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

A first aspect of the invention provides a composition having the formula A_(x)M_(y)M^(i) _(zi)O_(2−d), where

-   -   A is sodium or a mixed alkali metal including sodium as a major         constituent;     -   x>0;     -   M is a metal or germanium;     -   y>0;     -   M^(i), for i=1, 2, 3 . . . n, is a transition metal or an alkali         metal;     -   z_(i)≧0 for each i=1, 2, 3 . . . n;     -   0<d≦0.5;     -   the values of x, y, z_(i) and d are such as to maintain charge         neutrality; and     -   the values of x, y, z_(i) and d are such that x+y+Σz_(i)>2−d.

The present invention provides new compositions that display oxygen non-stoichiometry. The concept of oxygen non-stoichiometry will be briefly explained with regard to the composition ABO₂ in which element(s) A occupy one site and has an oxidation state of +1 and in which element(s) B occupy another site and has an oxidation state of +3. In an ideal (stoichiometric) material, the ratio of elements is A:B:O=1:1:2, and the oxygen has an oxidation state of −2 giving charge neutrality.

The ratio of elements may deviate from the ideal ABO₂ stoichiometry to give a composition with the non-stoichiometric formula ABO_(2-δ), with the average oxidation state of one or more elements contained within the B site being less than 3 to retain charge neutrality. However, the proportion of elements on the B site does not change, and there is variation only on the O site. So the relative proportions of elements within the material can be expressed as 1:1:(2−δ) and the composition may be thought of as “oxygen deficient” in that the oxygen content of (2−δ) is below the “ideal” value of 2. In a similar manner, it is in principle possible to obtain an “oxygen rich” composition with the formula ABO_(2+δ) with the average oxidation state of one or more elements contained within the B site being above 3 to retain charge neutrality.

The term “transition metal” as used herein includes the f-block lanthanides and actinides (sometimes referred to as the “inner transition metals”) as well as groups 3 to 12 of the periodic table.

M may be Ni, Mn, Fe, or Co.

M^(i), for i=1, 2, 3 . . . n, may be selected from the group consisting of: nickel, iron, cobalt, manganese, titanium, aluminium, magnesium and zirconium. However, the invention is not limited to this and M^(i) may alternatively be selected from the group consisting of: sodium, lithium, potassium, nickel, iron, cobalt, manganese, titanium, vanadium, niobium, tantalum, hafnium, chromium, molybdenum, tungsten, osmium, palladium, platinum, copper, silver, gold, zinc, cadmium, aluminium, scandium, yttrium, zirconium, technetium, rhenium, ruthenium, rhodium, iridium, mercury, gallium, indium, lead, bismuth, selenium, magnesium, calcium, beryllium, strontium, barium, boron, germanium, arsenic, antimony and tellurium.

Optionally, x>0.5. Optionally, x≧0.67.

Examples of compositions provided by the invention include:

-   NaNi_(0.5)Mn_(0.25)Zr_(0.25)O_(2−d) -   NaNi_(0.5)Ti_(0.25)Zr_(0.25)O_(2−d) -   NaNi_(0.5)Mn_(0.25)Ti_(0.25)O_(2−d) -   NaNi_(0.5)Mn_(0.45)Zr_(0.05)O_(2−d) -   NaNi_(0.5)Ti_(0.45)Zr_(0.05)O_(2−d) -   NaNi_(0.5)Mn_(0.4)Zr_(0.1)O_(2−d) -   NaNi_(0.5)Mn_(0.225)Ti_(0.225)Zr_(0.05)O_(2−d) -   NaNi_(0.5−x/2)Ti_(0.5−x/2)Al_(x)O_(2−d) -   NaNi_(0.5−x/2)Mn_(0.5−x/2)Al_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Mg_(x)Ti_(x)O_(2−d) -   NaNi_(0.333)Mn_(0.333)Mg_(0.167)Ti_(0.167)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Mg_(x/2)Ti_(x/2)Al_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Ca_(x)Ti_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Co_(x)Ti_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Cu_(x)Ti_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Zn_(x)Ti_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Mg_(x)Zr_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.25−x/2)Ca_(x)Ti_(0.25+x/2)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5)Ca_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−Y)Ca_(x)Ti_(Y)O_(2−d) -   NaNi_(0.5−x)Ti_(0.5−x)Mg_(x)Mn_(x)O_(2−d) -   NaNi_(0.5−x)Ti_(0.5−x)Ca_(x)Mn_(x)O_(2−d) -   NaNi_(0.5−x)Ti_(0.5−x)Cu_(x)Mn_(x)O_(2−d) -   NaNi_(0.5−x)Ti_(0.5−x)Co_(x)Mn_(x)O_(2−d) -   NaNi_(0.5−x)Ti_(0.5−x)Zn_(x)Mn_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5)Mg_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5)Ca_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5)Cu_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5)Co_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5)Zn_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−y)Mg_(x)Ti_(y)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−y)Ca_(x)Ti_(y)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−y)Cu_(x)Ti_(y)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−y)Co_(x)Ti_(y)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−y)Zn_(x)Ti_(y)O_(2−d) -   NaNi_(0.5−x)Mn_(0.25−x/2)Mg_(x)Ti_(0.25+x/2)O_(2−d) -   NaNi_(0.5−x)Mn_(0.25−x/2)Ca_(x)Ti_(0.25+x/2)O_(2−d) -   NaNi_(0.5−x)Mn_(0.25−x/2)Cu_(x)Ti_(0.25+x/2)O_(2−d) -   NaNi_(0.5−x)Mn_(0.25−x/2)Co_(x)Ti_(0.25+x/2)O_(2−d) -   NaNi_(0.5−x)Mn_(0.25−x/2)Zn_(x)Ti_(0.25+x/2)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Mg_(x/2)Ti_(x/2)Al_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Ca_(x/2)Ti_(x/2)Al_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Cu_(x/2)Ti_(x/2)Al_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Co_(x/2)Ti_(x/2)Al_(x)O_(2−d) -   NaNi_(0.5−x)Mn_(0.5−x)Zn_(x/2)Ti_(x/2)Al_(x)O_(2−d) -   NaMn_(x)Co_(1−(x+y))Fe_(y)O_(2−d) -   NaMn_(y)Co_(1−(x+y))Fe_(x)O_(2−d) -   NaMn_(y)Co_(1−(x+y))Fe_(x)O_(2−d) -   NaMn_(1−(x+y))Co_(y)Fe_(x)O_(2−d) -   NaMg_(x)Mn_(y)Co_(1−(x+y))O_(2−d) -   NaMg_(x)Mn_(y)Fe_(1−(x+y))O_(2−d) -   NaCa_(x)Mn_(y)Co_(1−(x+y))O_(2−d) -   NaCa_(x)Mn_(y)Fe_(1−(x+y))O_(2−d) -   NaMg_(x)Ti_(y)Co_(1−(x+y))O_(2−d) -   NaMg_(x)Ty_(x)Fe_(1−(x+y))O_(2−d) -   NaCa_(x)Ti_(y)Co_(1−(x+y))O_(2−d) -   NaCa_(x)Ti_(y)Co_(1−(x+y))O_(2−d) -   NaMg_(x)Mn_(y)Ti_(z)Co_(1−(x+y+z))O_(2−d) -   NaMg_(x)Mn_(y)Ti_(z)Fe_(1−(x+y+z))O_(2−d) -   NaCa_(x)Mn_(y)Ti_(z)Co_(1−(x+y+z))O_(2−d) -   NaMg_(x)Mn_(y)Ti_(z)Co_(1−(x+y+z))O_(2−d) -   NaCa_(x)Mn_(y)Ti_(z)Fe_(1−(x+y+z)/2)Co_(1−(x+y+z)/2)O_(2−d).

A second aspect of the invention provides an electrode comprising a composition according to the first aspect. The composition may form an active element of the electrode. An electrode of this aspect may be used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte material may comprise an aqueous electrolyte material, or it may comprise a non-aqueous electrolyte.

A third aspect of the invention provides an energy storage device comprising an electrode of the second aspect.

The energy storage device may be suitable for use as one or more of the following: a sodium and/or potassium ion cell; a sodium and/or potassium metal cell; a non-aqueous electrolyte sodium and/or potassium ion cell; and an aqueous electrolyte sodium and/or lithium and/or potassium ion cell.

A fourth aspect of the invention provides a rechargeable battery comprising at least one of an electrode of the second aspect and an energy storage device of the third aspect.

A fifth aspect of the invention provides an electrochemical device comprising at least one of an electrode of the second aspect and an energy storage device of the third aspect.

A sixth aspect of the invention provides an electrochromic device comprising at least one of an electrode of the second aspect and an energy storage device of the third aspect.

A seventh aspect of the invention provides an oxide ion conductor comprising a material having a composition according to the first aspect.

An eighth aspect of the invention provides a method of preparing a compound having the formula A_(x)M_(y)M^(i) _(zi)O_(2−d), where A is an sodium or a mixed alkali metal including sodium, x>0, M is a transition metal, y>0, M^(i), for i=1, 2, 3 . . . n is a transition metal or an alkali metal, z_(i)≧0 for each i=1, 2, 3 . . . n, 0<d≦0.5, and the values of x, y, z_(i) and d are such as to maintain charge neutrality, the method comprising:

a) mixing starting materials together, b) heating the mixed starting materials at a temperature of between 400° C. and 1500° C., to obtain an oxygen-deficient reaction product having the formula A_(x)M_(y)M^(i) _(zi)O_(2−d′); and c) cooling the reaction product, or allowing the reaction product to cool, under conditions that prevent significant re-incorporation of oxygen into the oxygen-deficient reaction product.

Since the reaction product is cooled, or allowed to cool, under conditions that prevent significant re-incorporation of oxygen into the oxygen-deficient reaction product, the end product is an oxygen deficient composition having the formula A_(x)M_(y)M^(i) _(zi)O_(2−d). (It should be noted that some slight re-incorporation of oxygen may occur, so that the oxygen fraction 2−d of the end product may not be exactly equal to the oxygen fraction 2−d′ of the oxygen-deficient reaction product obtained by heating the starting materials.)

The present invention can provides a bulk material with an oxygen deficient composition. In contrast to some prior techniques can only provide mild levels of reduction (that is, mild levels of oxygen loss) on the surface of the sample, and cannot provide a bulk oxygen deficient material.

The mixed starting materials may be heated for more than 30 seconds and/or may be heated for less than 200 hours, and optionally may be heated for more than 2 hours and/or may be heated for less than 200 hours.

Cooling the reaction product may comprise cooling the reaction product in one of an inert atmosphere and a reducing atmosphere.

Cooling the reaction product may comprise cooling the reaction product in an inert atmosphere, the inert atmosphere being, or consisting substantially of, one or more inert gases. Examples of suitable inert gases include nitrogen, and argon and other of the noble gases.

Alternatively, cooling the reaction product may comprise cooling the reaction product in a reducing atmosphere such as, for example, an atmosphere of hydrogen in nitrogen. However, the invention does not require cooling the reaction product in a reducing atmosphere, and requires only that the cooling is carried out under conditions that prevent significant re-incorporation of oxygen.

Heating the mixed starting materials to obtain an oxygen-deficient reaction product may comprise heating the mixed starting materials in an oxidising atmosphere.

Heating the mixed starting materials to obtain an oxygen-deficient reaction product may comprise heating the mixed starting materials in air.

Compositions of the invention include compounds that are oxygen deficient. Further the oxidation states may or may not be integers i.e. they may be whole numbers or fractions or a combination of whole numbers and fractions and may be averaged over different crystallographic sites in the material.

The present Applicant has found that not only are the oxidation states of the metal constituents in the compounds of the present invention a critical feature to the production of highly electrochemically active compounds but they have also confirmed that having particular transition metal constituents allows variable oxidation states (i.e. oxidation states which are not integers) in the same crystalline structure of the compound. It is known that that there are several possible layered structural forms which alkali metal/metal/oxides may adopt, including O3, P3 and P2. In this application the Applicant has shown that the oxidation states for the metal constituents can allow oxidation states which are not integers to be stabilised in the O3 structural form via the loss of oxygen from the material. The magnitude of the loss of oxygen may also be controlled by tailoring the synthesis of these materials. The applicant has also noted several notable benefits in the application of these materials in electrochemical devices. Materials which show oxygen deficiency generally show lower irreversibility on alkali metal intercalation and de intercalation, they also show similar capacity retention and similar intercalation potentials. The slightly lower oxygen content also results in dilation of the unit cell and realises a smaller volume change on electrochemical cycling, leading to improvements in cell lifetime.

Preferred compounds of the present invention include:

-   e.g. NaNi_(0.5)MnO_(2−d) -   NaCo_(0.5)Mn_(0.5)O_(2−d) -   NaCo_(0.5) Mn_(0.25)Ti_(0.25)O_(2−d) -   NaNi_(0.33)Co_(0.33)Mn_(0.33)O_(2−d) -   NaNi_(0.33)Fe_(0.33)Mn_(0.33)O_(2−d)

The magnitude of d can be controlled.

Additionally preferred electrodes of the present invention comprise active compounds defined above in which d>0.01.

It is particularly advantageous if the electrode comprises active compounds in which transition metals of oxidation states +1, +2 and +4 are combined on the metal site.

The presence of oxygen deficiency is particularly advantageous to improve the electrochemical irreversibility on cycling; resulting in the active materials which are capable of being charged and recharged numerous times with a lower drop in capacity on the first electrochemical cycle. The active materials comprising oxygen deficiency are also advantageous because the reduction of Oxygen content does not appear to have any adverse effect on the intercalation potentials of the material, but yields a few percent gain in gravimetric energy density.

Preferred electrodes of the present invention comprise active compounds selected from one or more of:

-   NaNi_(0.5)Mn_(0.25)Zr_(0.25)O_(2−d) -   NaNi_(0.5)Ti_(0.25)Zr_(0.25)O_(2−d) -   NaNi_(0.5)Mn_(0.25)Ti_(0.25)O_(2−d) -   NaNi_(0.333)Mn_(0.333)Mg_(0.167)Ti_(0.167)O_(2−d) -   NaCo_(0.5)Mn_(0.5)O_(2−d) -   NaCo_(0.5) Mn_(0.25)Ti_(0.25)O_(2−d) -   NaNi_(0.33)Co_(0.33)Mn_(0.33)O_(2−d) -   NaNi_(0.33)Fe_(0.33)Mn_(0.33)O_(2−d)

The electrodes according to the present invention are suitable for use in many different applications, for example energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices.

Advantageously, the electrodes according to the invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s) or mixtures thereof.

In a further aspect, the present invention provides an energy storage device that utilises an electrode comprising the active materials described above, and particularly an energy storage device for use as one or more of the following: a sodium and/or lithium and/or potassium ion cell; a sodium and/or lithium and/or potassium metal cell; a non-aqueous electrolyte sodium and/or potassium ion; an aqueous electrolyte sodium and/or lithium and/or potassium ion cell.

The novel compounds of the present invention may be prepared using any known and/or convenient method. For example, the precursor materials may be heated in a furnace so as to facilitate a solid state reaction process. They may be further processed at elevated temperatures to induce, reduce or increase the magnitude of oxygen deficiency in the crystal structure.

A further aspect of the present invention provides a particularly advantageous method for the preparation of the compounds described above comprising the steps of:

a) mixing the starting materials together, preferably intimately mixing the starting materials together and further preferably pressing the mixed starting materials into a pellet; b) heating the mixed starting materials in a furnace at a temperature of between 400° C. and 1500° C., preferably a temperature of between 500° C. and 1200° C., for between 2 and 200 hours. Heating may be undertaken using; a static atmosphere, a flowing gas or a purging gas. Precursor materials may be heated under, oxygen containing atmospheres, reducing atmospheres or inert atmospheres; and c) allowing the reaction product to cool. Cooling may also be undertaken using; a static atmosphere, a flowing gas or a purging gas. Precursor materials may be cooled under, oxygen containing atmospheres, reducing atmospheres or inert atmospheres.

Preferably the reaction is conducted under an atmosphere of ambient air, and alternatively under an inert gas.

DESCRIPTION OF DRAWINGS

FIG. 1(A) shows Powder X-ray diffraction pattern of Na Ni_(0.5)Ti_(0.5)O₂ prepared according to Example 1.

FIG. 1(B) shows the TGA-STA data obtained for Na Ni_(0.5)Ti_(0.5)O₂ (Example 1) heated to a temperature of 800° C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

FIG. 1(C) Powder X-ray diffraction patterns of Na Ni_(0.5)Ti_(0.5)O₂ after being heated to 800° C. and cooled in either air or nitrogen.

FIG. 1(D) shows the first three charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) for the cathode material prepared according to Example 1 after heating to 800° C. under a constant flow of air (Solid line) or under a constant flow of nitrogen (dashed line).

FIG. 1(E) shows the Differential Capacity Profiles for the 1^(st) charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) for the cathode material prepared according to Example 1 heated to a temperature of 800° C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

FIG. 2(A) shows Powder X-ray diffraction pattern of Na_(0.95)Ni_(0.3167)Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O₂ prepared according to Example 2.

FIG. 2(B) shows the TGA-STA data obtained for Na_(0.95)Ni_(0.3167)Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O₂ (Example 2) heated to a temperature of 800° C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

FIG. 2(C) Powder X-ray diffraction patterns of Na_(0.95)Ni_(0.3167)Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O₂ after being heated to 800° C. and cooled in either air or nitrogen.

FIG. 2(D) shows the first eight charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) for the cathode material prepared according to Example 2 after heating to 800° C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

FIG. 2(E) shows the cycle life (Cathode specific capacity [mAh/g] vs cycle number) for the cathode material prepared according to Example 2 after heating to 800° C. C under a constant flow of air (circles) or under a constant flow of nitrogen (triangles). Dotted line represents discharge and solid line represents charge

FIG. 2(F) shows the Differential Capacity Profiles for the 1^(st) charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) for the cathode material prepared according to Example 2 heated to a temperature of 800° C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

FIG. 3(A) shows Powder X-ray diffraction pattern of Na_(0.95)Ni_(0.3167)Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O₂ prepared according to Example 3.

FIG. 3(B) shows the TGA-STA data obtained for Na_(0.95)Ni_(0.3167)Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O₂ (Example 3) heated to a temperature of 800° C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

FIG. 3(C) Powder X-ray diffraction patterns of Na_(0.95)Ni_(0.3167)Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O₂ after being heated to 800° C. and cooled in either air or nitrogen.

FIG. 3(D) shows the first two charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) for the cathode material prepared according to Example 3 after heating to 800° C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

FIG. 3(E) shows the Differential Capacity Profiles for the 1^(st) charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) for the cathode material prepared according to Example 3 heated to a temperature of 800° C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

DETAILED DESCRIPTION

The materials according to the present invention are prepared using the following generic method:

Synthesis Method:

The required amounts of the precursor materials are intimately mixed together and either pressed into a pellet or retained as a free flowing powder. The resulting mixture is then heated in a tube furnace or a chamber furnace using either an ambient air atmosphere, or a flowing inert atmosphere (e.g. argon or nitrogen), at a furnace temperature of between 400° C. and 1500° C. until reaction product forms; for some materials a single heating step is used and for others (as indicated below in Table 1) more than one heating step is used. Different cooling protocols can be used to induce oxygen non-stoichiometry in the materials. Sample may be heated and cooled under different atmospheres as indicated in table 1. When cool, the reaction product is removed from the furnace and ground into a powder prior to characterisation.

Using the above generic method, active materials were prepared, Examples 1 to 3, as summarised below in Table 1:

TARGET STARTING Example COMPOUND MATERIALS FURNACE CONDITIONS 1 NaNi_(0.5)Ti_(0.5)O_(2-d) TiO₂ 1^(st) firing (d = 0) = 900° C., 8 h, Na₂CO₃ Air NiCO₃ 2^(nd) firing (d > 0) = 800° C., 1 h, N₂ 2 Na_(0.95)Ni_(0.3167) Na₂CO₃ Stoichiometric (900° C., 10 h, Air) Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O_(2-d) NiCO₃ Oxygen deficient ((900° C., 1 h, I under N2) MnCO₃ MgCO₃ TiO₂ 3 Na_(0.95)Ni_(0.3167) Na₂CO₃ Stoichiometric (900° C., 10 h, Air) Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O_(2-d) NiCO₃ Oxygen deficient ((900° C., 10 h, Air and MnCO₃ cool under N2) MgCO₃ TiO₂

One example of a method of the invention is an indirect route in which we first mix the precursors and then fire the mixture to produce a stoichiometric layered oxide.

To form the oxygen non-stoichiometric form a secondary processing step is used. The secondary processing step can take one of two forms, the material may be re heated under air to a temperature close to the formation temperature of the material and cooled under a flow of nitrogen. This method relies on preventing the re-uptake of oxygen in the material. A broadly similar method can also be used in which the secondary processing step can be undertake under an inert atmosphere such as an atmosphere consisting, or consisting substantially of, an inert gas or a mixture of inert gases. Examples of suitable inert gases include nitrogen, and argon and the other noble gases. That is, in one example, both the heating and cooling steps can be conducted under a nitrogen atmosphere to yield the non-stoichiometric form of the oxide.

Another example of a method of the invention is a single step process in which we mix the precursor materials together, heat the mixture to an appropriate temperature and holding for a specified time to allow the layered oxide to form. At this stage the layered oxide will be in a metastable state. Simply changing the atmosphere to an inert atmosphere (for example by putting the reaction chamber under a nitrogen atmosphere) at the end of the formation of the material and cooling to room temperature yield the oxygen deficient form of the material without the need for secondary processing.

Generic Procedure to Make a Sodium Metal Electrochemical Test Cell:

Electrochemical cells were prepared using conventional electrochemical testing techniques. Materials were either tested as powder, pressed pellets or as cast electrodes, each testing methodology used is highlighted alongside the example materials. To prepare an electrode of the test material the sample was prepared using a solvent-casting technique, from a slurry containing the active material, conductive carbon, binder and solvent. The conductive carbon used is Super P C65 (Timcal). PVdF (e.g. Kynar) is used as the binder, and NMP (N-Methyl-2-pyrrolidone, Anhydrous, Sigma, UK) is used as the solvent. The slurry is then cast onto an aluminium current collector using the Doctor-blade technique. The electrode is then dried under Vacuum at about 80-120° C. The electrode film contains the following components, expressed in percent by weight: 75% active material, 18% Super P carbon, and 7% Kynar binder. Optionally, this ratio can be varied to optimise the electrode properties such as, adhesion, resistivity and porosity. The electrolyte comprises a 0.5 or 1.0 M solution of NaClO₄ in propylene carbonate (PC), and can also be any suitable or known electrolyte or mixture thereof. A glass fibre separator (e.g. Whatman, GF/A) or a porous polypropylene separator (e.g. Celgard 2400) wetted by the electrolyte is interposed between the positive and negative electrodes forming the electrochemical test cell. Typically, cells were symmetrically charged and discharged galvanostatically at a rate of 5-10 mA/g.

The materials described herein can also be tested as powders, where the active material is mixed with a conductive additive, either by hand mixing or in a ball mill. The conductive carbon used is Super P C65 (Timcal). The electro active mixture contains the following components, expressed in percent by weight: 80% active material, 20% Super P carbon, this ratio can be varied to optimise the mixtures properties such as, resistivity and porosity. The electrolyte comprises a 0.5 or 1.0 M solution of NaClO₄ in propylene carbonate (PC). A glass fibre separator (e.g. Whatman, GF/A) or a porous polypropylene separator (e.g. Celgard 2400) wetted by the electrolyte is interposed between the positive and negative electrodes forming the electrochemical test cell. Typically, cells were symmetrically charged and discharged galvanostatically at a rate of 5-10 mA/g.

Alternatively, materials described herein may also be tested as pressed pellets, where the active material is mixed with a conductive additive and a polymer binder, either by hand mixing or in a ball mill. The conductive carbon used is Super P C65 (Timcal). The electro active mixture contains the following components, expressed in percent by weight: 80% active material, 10% Super P Carbon, and 10% binder (PVdF or similar), this ratio can be varied to optimise the mixtures properties such as, resistivity, porosity and wetting behaviour of the pellet. The electrolyte comprises a 0.5 or 1.0 M solution of NaClO₄ in propylene carbonate (PC). A glass fibre separator (e.g. Whatman, GF/A) or a porous polypropylene separator (e.g. Celgard 2400) wetted by the electrolyte is interposed between the positive and negative electrodes forming the electrochemical test cell. Typically, cells were symmetrically charged and discharged galvanostatically at a rate of 5-10 mA/g.

Cell Testing:

Electrochemical cells of materials prepared according to the procedures outlined in Table 1 were tested using Constant Current Cycling Techniques.

The cell was cycled at a given current density (ca. 5-10 mA/g) between pre-set voltage limits as deemed appropriate for the material under test. A commercial battery cycler from Maccor Inc. (Tulsa, Okla., USA) was used. Cells were charged symmetrically between the upper and lower voltage limits at a constant current density. On charge sodium ions are extracted from the cathode and migrate to the anode. On discharge the reverse process occurs and Sodium ions are re-inserted into the cathode material.

Structural Characterisation:

All of the product materials were analysed by X-ray diffraction techniques using a Bruker D2 phaser powder diffractometer (fitted with a Lynxeye™ detector) to confirm that the desired target materials had been prepared, and also to establish the phase purity of the products and to determine the types of impurities present. From this information it is possible to determine the unit cell lattice parameters.

The operating conditions used to obtain the powder diffraction patterns illustrated, are as follows:

Range: 2θ=10°-90° X-ray Wavelength=1.5418 Å (Angstoms) (Cu Kα)

Step size: 2θ=0.02 Speed: 1.5 seconds/step

Diffraction patterns were collected using sample holders which could allow measurement of diffraction under an inert atmosphere. The sample holder contributes to the observed diffraction patterns with large peaks centered at ca. 32°=2θ and ca. 50°=2θ and other smooth peak features can also be observed.

Quantification of Oxygen Loss and Uptake:

The loss or uptake of oxygen could be readily quantified using TGA-STA (Thermogravimetric analysis-simultaneous thermal analysis) using a Perkin Elmer STA 6000 equipped with a passivated Al₂O₃ crucible. To quantify the oxygen loss or uptake in a layered oxide sample the sample was reheated to a temperature less than or equal to the formation temperature. In a typical experiment the sample was heated at a rate of 20° C. min⁻¹ to a temperature less than or equal to the formation temperature, the sample was held at temperature for a period in the range 60 s-1 h to allow equilibrium of any metastable state, the sample was cooled at a rate of 20° C. min⁻¹ to room temperature. The heating and cooling protocol varied by sample as did the combination of flowing gasses and the point at which gas flows were changed. For example, to demonstrate oxygen loss in a stoichiometric sample the sample was heated and cooled under a constant nitrogen flow throughout the entire cycle. Similarly, to demonstrate oxygen uptake in an oxygen deficient sample the sample cycle was completed under a constant flow of air. Control experiments were performed to confirm complete oxygen re-uptake or retention of oxygen deficiency by heating under an inert gas followed by cooling under oxygen or heating under air followed by heating under an inert gas, respectively.

DETAILED DESCRIPTION OF THE INVENTION

Results:

The present Applicant has found that not only are the oxidation states of the metal constituents in the compounds of the present invention a critical feature to the production of highly electrochemically active compounds but they have also confirmed that having particular transition metal constituents allows variable oxidation states (i.e. oxidation states which are not integers) in the same crystalline structure of the compound. It is known that that there are several possible layered structural forms which alkali metal/metal/oxides may adopt, including O3, P3 and P2. The Applicant has shown that the oxidation states for the metal constituents can allow oxidation states which are not integers to be stabilised in many structural forms including O2, P3 and P2 via the loss of oxygen from the material. This is achieved through incorporating a reducible transition metal within the material composition. The magnitude of the loss of oxygen may also be controlled by tailoring the synthesis of these materials. The applicant has also noted several benefits in the application of these materials in electrochemical devices. Materials which show oxygen deficiency generally show lower irreversibility on alkali metal intercalation and de intercalation, they also show similar capacity retention and similar intercalation potentials. The slightly lower oxygen content also results in dilation of the unit cell and realises a smaller volume change on electrochemical cycling, leading to improvements in cell capacity retention.

The invention will now be described with reference to the example materials.

The invention relates to materials which have lost oxygen from their ideal stoichiometry. In the simplest embodiment of the invention oxygen loss may be induced in a stoichiometric layered oxide by post processing. This aspect of the present invention will now be described in reference to examples 1-3.

With reference to Example 1, FIG. 1A shows the Powder X-ray diffraction pattern of stoichiometric NaNi_(0.5) Ti_(0.5)O₂ showing the formation of an O3 layered oxide phase produced as described in Example 1 for the formation of the stoichiometric layered. In this material a mass loss of 0.41% was realised when post processed by heating to 800° C. and cooling under N₂, this equates to a stoichiometry if the mass loss is associated with oxygen of NaNi_(0.5)Ti_(0.5)O_(1.97).

The first four charge-discharge voltage profiles (Na-ion half cell Voltage against a sodium metal anode [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) for example 1 are shown in FIG. 1 D in which the sample produced by post processing in air and under N₂ are compared. It can be seen FIG. 1D that inducing oxygen deficiency within this material leads to an increase in the cycling capacity 120 mAh/g and 110 mAh/g for the non-stoichiometric and stoichiometric samples. The observed irreversibility in this material was similar between the stoichiometric and non-stoichiometric sample. The average voltage of each cell is similar at 3.03 and 2.88 V vs Na/Na+ for the sample post processed in nitrogen and air, respectively. Calculation of specific energy density yields 364 and 317 Wh/kg for the sample post processed in nitrogen and air, respectively. The increase in specific energy density shown in the oxygen deficient materials is a distinct advantage, in for example, an electrochemical cell and for industrial application of these materials. The Differential Capacity Profiles for the 1^(st) charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) are shown in FIG. 1 E in which the reduction of irreversibility in the material can be attributed to a peak centered at 4.18 V vs Na/Na+, this is usually attributed to oxygen loss in O3 layered oxide materials. From the differential capacity plot shown in FIG. 1 E we believe that oxygen deficient materials show lower oxygen loss on cycling to high voltages which may also be beneficial in manufacture of full cells based on these materials. We have demonstrated that this oxygen loss consistently results in a reduced first cycle loss and a slight increase in the average potential of a cell constructed from oxygen deficient layered oxides.

With reference to Example 2. The data shown in FIG. 2A shows the Powder X-ray diffraction pattern of stoichiometric Na_(0.95)Ni_(0.3167)Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O₂ showing the formation of an O3 layered oxide phase produced as described in Example 2 for the formation of a stoichiometric layered oxide. Post processing of this material leads to the loss of oxygen when post processing via heating under an inert atmosphere is undertaken. Oxygen loss in Na_(0.95)Ni_(0.3167)Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O₂ was demonstrated using TGA-STA by reheating the sample to a temperature of 900° under a constant flow of N₂. The mass loss associated with reheating the material under air and under N₂ are compared in FIG. 2B. It can be seen that loss of oxygen occurs in the sample heated in N₂ leading to an overall mass loss. The mass loss observed in the sample heated under nitrogen is 1.15% this equates to a stoichiometry if the mass loss is associated with oxygen of Na_(0.95)Ni_(0.3167)Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O_(1.907) with a formula weight of 102 g/mol.

Electrochemically these materials show a benefit over samples which do not show non-stoichiometry in oxygen. The eight charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) are shown in FIG. 2 D in which the sample produced by post processing in air and under N₂ are compared. It can be seen FIG. 2 D that inducing oxygen deficiency within this material leads to an slight increase in the cycling capacity. However, in this material there is no difference in the in the irreversible capacity loss which is 26% in the N2 treated sample and 31% in the air heated sample with similar capacity fade over the first few cycles. The average voltage of each cell is similar at 2.56 and 2.95 V vs Na/Na+ for the sample post processed in air and nitrogen, respectively. Calculation of specific energy density of the materials yields values of 333 and 480 Wh/kg for sample post processed in air and nitrogen, respectively. Showing that oxygen non-stoichiometry in this material leads to a specific energy density gain verses the stoichiometric oxide. FIG. 2 E compares the cycling capacities and capacity retention of the materials produced from Example 2 and provides a further indication of the slight reduction in first cycle loss. The reduction of first cycle loss is also demonstrated in FIG. 2 F in which the differential capacity plot of the first electrochemical cycle is shown.

In a second embodiment of the invention oxygen loss maybe induced in materials through a single step synthesis process by tailoring the synthesis conditions. The single step synthesis of these materials will now be described with reference to Example 3.

Example 3 demonstrates that oxygen non-stoichiometric materials may be produced in a single processing step which yields further industrial advantages such as reduced material processing costs. FIG. 3A shows the Powder X-ray diffraction pattern of oxygen deficient Na_(0.95)Ni_(0.3167)Mn_(0.3167)Mg_(0.1583)Ti_(0.2018)O_(2−d) showing the formation of an O3 layered oxide phase through a single step synthesis route. To quantify the magnitude of oxygen loss in the material a slightly modified TGA-STA analysis was performed in which mass gain on reheating under air was quantified and a control experiment was done under an inert atmosphere as shown in FIG. 3 B. In this material a mass increase of 0.77% was realised when the sample was re-heated to 800° C. under an air atmosphere, this equates to a stoichiometry if the mass increase is associated with oxygen of NaNi_(0.25)Na_(0.17)Mn_(0.166)Ti_(0.416)O_(1.95). It can be seen that no difference in sample mass is observed in the sample heated under nitrogen. To confirm that no structural transitions occurred in the sample post processed in air a XRD pattern of the material is shown in FIG. 3C in which it can be seen that no structural transitions occurs with the uptake of oxygen.

The electrochemical properties of the material produced directly were compared to those obtained from a stoichiometric material produced using the method described in Table 1. The first three charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) are shown in FIG. 3 D in which the sample produced by post processing in air and under N₂ are compared. It can be seen FIG. 3 D that inducing oxygen deficiency within this material leads to a slight increase in the cycling capacity, and a very slight reduction in the irreversible capacity loss on the first cycle. These observations are consistent with those taken directly from Examples 1 and 2. The average voltage of the directly produced sample is also similar to that obtained for the stoichiometric sample at 3.04 and 2.62 V vs Na/Na+ for the directly produced sample and the sample post processed in air, respectively. Calculation of specific energy density yields 491 and 388 Wh/kg for the sample produced directly and the sample post processed in air, respectively. The increase in specific energy density shown in the oxygen deficient materials is a distinct advantage in the exploitation of such materials in energy storage devices. The Differential Capacity Profiles for the 1^(st) charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) are shown in FIG. 3 E in which the reduction of irreversibility in the material can be attributed to a peak centered at 4.15 V vs Na/Na+, this is usually attributed to oxygen loss in O3 layered oxide materials. Further suggesting oxygen deficient materials evolve less oxygen when charged to high potentials which may also be beneficial in manufacture of full cells based on these materials.

INDUSTRIAL APPLICABILITY

Electrodes according to the present invention are suitable for use in many different applications, energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices. Advantageously the electrodes according to the invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise of either aqueous electrolyte or non-aqueous electrolytes or a mixture thereof. 

1. A method of preparing a compound having the formula A_(x)M_(y)M^(i) _(zi)O_(2−d), where A is an sodium or a mixed alkali metal including sodium, x>0, M is a transition metal, y>0, M^(i), for i=1, 2, 3 . . . n is a transition metal or an alkali metal, z_(i)≧0 for each i=1, 2, 3 . . . n, 0<d≦0.5, and the values of x, y, z_(i) and d are such as to maintain charge neutrality, the method comprising: a) mixing starting materials together, b) heating the mixed starting materials at a temperature of between 400° C. and 1500° C., to obtain an oxygen-deficient reaction product having the formula A_(x)M_(y)M^(i) _(zi)O_(2−d′); and c) cooling the reaction product, or allowing the reaction product to cool, under conditions that prevent significant re-incorporation of oxygen into the oxygen-deficient reaction product.
 2. A method as claimed in claim 1 wherein cooling the reaction product comprises cooling the reaction product in one of an inert atmosphere and a reducing atmosphere.
 3. A method as claimed in claim 1 wherein cooling the reaction product comprises cooling the reaction product in an inert atmosphere, the inert atmosphere being, or consisting substantially of, one or more inert gases.
 4. A method as claimed in claim 1 wherein heating the mixed starting materials to obtain an oxygen-deficient reaction product comprises heating the mixed starting materials in an oxidising atmosphere.
 5. A method as claimed in claim 1 wherein heating the mixed starting materials to obtain an oxygen-deficient reaction product comprises heating the mixed starting materials in air. 